The present invention relates generally to a reprographic printing machine. More specifically, the present invention pertains to an apparatus for assisting the transfer of a developed image from an imaging surface, such as a photoconductive surface or intermediate image transfer surface, to a print sheet, such as paper, by optimizing the contact between the print sheet and the imaging surface. The present invention also pertains to such a transfer assist apparatus including a variable length transfer assist blade that may be adjusted for a plurality of different size print sheets.
FIG. 1 is a schematic illustration of a typical electrophotographic printing machine 10 that may employ a transfer assist blade according to the present invention (not shown in FIG. 1). The illustrated printing machine 10 includes a conventional photoconductive layer or light sensitive surface 12 on a conductive backing in the form of a photoconductive belt 14. The photoconductive belt 14 is mounted on a plurality of rollers journaled in a machine frame (not shown), in order to rotate the photoconductive belt 14 and cause the photoconductive layer 12 to pass sequentially through a plurality of reprographic process stations A through E.
The several generally conventional processing stations A through E in the path of movement of the photoconductive layer 12 may be as follows. A charging station A, where the photoconductive layer 12 of the photoconductive belt 14 is uniformly charged. An exposure station B, where a light or radiation pattern of a document to be printed is projected onto the photoconductive layer 12 to expose and discharge select areas of the photoconductive layer 12 to form a latent image thereon. A developing station C, where developer material is applied to the photoconductive layer 12 of the photoconductive belt 14 to generate a toner image on the photoconductive layer 12. A transfer station D, where the toner image is electrostatically transferred from the photoconductive surface to a print sheet 30. Finally, a cleaning station E, where the photoconductive surface is brushed or otherwise cleared of residual toner particles remaining thereon after image transfer.
In order to generate multi-color prints, there may be a group of processing stations A through E for each of a plurality of colors. For example, there may be a group of stations A through E for each of yellow, cyan, magenta and black. One method of generating multicolor prints is to arrange all of the color stations around a single photoreceptor and generate a toner image on the photoreceptor for each color, one color at a time. After each individual color toner image is formed on the photoreceptor, it is transferred to an intermediate transfer surface before the next color toner image is generated. This is repeated for each color, thereby building up a full color toner image on the intermediate transfer surface. The full color toner image is then transferred from the intermediate transfer surface to the print sheet. The intermediate transfer surface may be formed on an intermediate transfer belt, roll, drum or other suitable structure. Alternatively, a separate photoreceptor may be provided for each color. In which case, each color toner image is formed on the corresponding photoreceptor and transferred to the intermediate transfer surface, thereby creating a multi-color toner image on the intermediate transfer surface. The multi-color toner image is then transferred from the intermediate transfer surface to the print sheet.
Another method of generating full color prints is to arrange all of the color processing stations around a single photoreceptor and form all of the color toner images, one on top of each other, during a single rotation of the photoreceptor. The full color toner image may then be transferred from the photoreceptor to the print sheet, eliminating the need for an intermediate transfer surface.
Print sheets 30, such as paper or other print substrate, supplied from a sheet feeding tray or sheet feeding module 16, are fed by a series of sheet feeding rollers and guide rails to the transfer station D. At the transfer station D, the developed toner image is transferred from the photoconductive belt 14 (or intermediate transfer surface) to the print sheet 30. The print sheet 30 is then stripped from the photoconductive belt 14 by a sheet stripper and transported to a fusing station F, where a fuser 20 fuses the toner image onto the print sheet 30 in a known manner. The print sheet 30, which now has an image fused to a first face thereof, is then transported by a plurality of rollers to an output tray or stacking module 26 for one-sided or simplex copying. It will be appreciated that the print sheet may pass directly into the stacking module 26. It will also be appreciated that the print sheet may be inverted prior to entering the stacking module 26 or may be inverted and returned to the developing station C for duplex printing.
The various machine operations are regulated by a controller which is preferably a programmable microprocessor capable of managing all of the machine functions and subsystems. Programming conventional or general purpose microprocessors to execute imaging, printing, document, and sheet handling control functions with software instructions and logic is well known and commonplace in the art. Such programming or software will, of course, vary, depending on the particular machine configuration, functions, software type, and microprocessor or other computer system utilized. Those of skill in the software and/or computer arts can readily program the microprocessor and/or otherwise generate the necessary programming from functional descriptions, such as those provided herein, or from general knowledge of conventional functions together with general knowledge in the software and computer arts without undue experimentation. The operation of the exemplary systems described herein may be accomplished by conventional user interface control inputs selected by the operator from the printing machine consoles. Conventional sheet path sensors or switches may be utilized to keep track of the position of documents and print sheets in the machine 10.
The electrophotographic printing process and machine 10 described above, and variations thereof, are well known and are commonly used for light lens copying and digital printing and photocopying. In digital printing and photocopying processes, a latent image is produced by modulating a laser beam or by selectively energizing light emitting diodes in an array of diodes. A digital original may be created digitally in any known manner, or may be a digital image of a hard copy that was previously scanned, digitized and stored in memory. In ionographic printing and reproduction, a charge is selectively deposited on a charge retentive surface in response to an electronically generated or stored image. It should be understood that a drum photoreceptor, or flash exposure may be alternatively employed.
The process of transferring charged toner particles from an image bearing member, such as the photoconductive belt or an intermediate transfer member to a print sheet is accomplished in a reprographic machine by overcoming the adhesive and electrostatic forces holding the toner particles to the image bearing member. This has been accomplished, for example, via electrostatic induction using a corona generating device. The print sheet is placed in direct contact with the developed toner image on the image bearing member, while the reverse side of the print sheet is exposed to a corona discharge. The corona discharge generates ions having a polarity opposite that of the toner particles on the image bearing member. The ions electrostatically attract the toner particles from the image bearing member and into contact with the print sheet, thereby transferring the toner particles from the image bearing member to the print sheet. Other forces, such as mechanical pressure or vibratory energy, have also been used to support and enhance the electrostatic transfer process.
To achieve substantially complete transfer of the developed image to the print sheet, it is necessary for the print sheet to be in intimate uniform contact with the image bearing member. However, the interface between the image bearing member and the print sheet is rarely uniform. Print sheets that have been mishandled, left exposed to the environment, or previously passed through a fixing operation (e.g., heat and/or pressure fusing) tend to be non-flat or uneven. An uneven print sheet makes uneven contact with the image bearing member. In the event that the print sheet is wrinkled, the print sheet will not be in continuous intimate contact with the image bearing member. Wrinkles in the print sheet cause spaces or air gaps to materialize between the developed toner particle image on the image bearing member and the print sheet. When spaces or gaps exist between the developed image and the print sheet, various problems may result. For example, there is a tendency for toner particle not to transfer across the gaps, causing variable transfer efficiency and creating areas of low toner particle transfer or even no transfer. A phenomenon known as image transfer deletion. Clearly, image transfer deletion is undesirable in that portions of the desired image may not be appropriately reproduced on the print sheet.
One known approach for curing the transfer deletion problem is illustrated in U.S. Pat. No. 5,247,335 to Smith et al., which discloses a flexible blade member, or so-called transfer assist blade. A solenoid-activated lever arm moves the transfer assist blade from a non-operative position spaced from the print sheet, to an operative position in contact with the print sheet. When in the operative position, the transfer assist blade presses the print sheet into contact with a developed image on a photoconductive surface, thereby substantially eliminating wrinkles in the print sheet and gaps between the print sheet and the photoconductive surface.
U.S. Pat. No. 4,947,214 to Baxendell et al. and U.S. Pat. No. 5,227,852 to Smith et al. each disclose a transfer assist blade formed of two separately actuated segments, thereby providing a variable length transfer assist blade. A first of the segments is actuated when an 11 inch sheet is passing through a developing station. Both segments are actuated when a 14 inch sheet is passing through the developing station. A separate blade actuating motor and linkage arrangement is provided for each blade segment.
U.S. Pat. No. 5,300,993 to Vetromile and U.S. Pat. No. 5,300,944 to Gross et al. each disclose a variable length transfer assist blade apparatus formed of a plurality of blade segments. In order to accommodate print sheets of a plurality of cross-process dimensions, varying numbers of the blade segments are selectively actuated into and out of their operative position in contact with the print sheet by a cam shaft. The cam shaft has a plurality of lobes or cam segments of varying length. The cam shaft is rotated so that the lobe having a length that corresponds to the desired actuated or unactuated length of the transfer assist blade presses against the blade segments. Thus, the cam shaft deflects the desired number of blade segments into (see Gross et al.) or out of (see Vetromile et al.) contact with the photoconductive surface. The cam shaft disclosed by Vetromile et al. and Gross et al. enables the selective deflection of varying numbers of blade segments with a single drive motor that rotates the cam shaft.
For obvious reasons, it is desirable that the size or footprint of modern reprographic printing machines be as small as possible. As the size of the reprographic machines is reduced, the space available in the printing machine for the transfer assist blade and associated mechanisms is similarly reduced. Furthermore, the space between the corona generating device and the photoconductive surface is extremely limited. The space limitations are multiplied in full color xerographic machines. A color xerographic printing machine typically has a plurality of sets of charging, developing and transfer stations, for example, one set for each of yellow, cyan, magenta and black, packed into the available interior space. Due to the limited space available in reprographic printing machines, the prior art variable length transfer assist blade systems are limited to providing segmented transfer assist blades having lengths corresponding to a relatively limited number of discrete sheet dimensions.
Many of the existing variable length transfer assist blade devices require a separate actuation motor and linkage for each blade segment. As the number of blade segments is increased, the number of motors and links is also increased. As a result, the cost and complexity of the system increases dramatically as the number of blade segments is increased. Furthermore, only a limited number of motors and associated linkage mechanisms will fit within the available space. On the other hand, existing devices that employ a single cam shaft to actuate all of the transfer assist blade segments eliminate the need for a separate drive motor and linkage for each blade segment. As the number of blade segments is increased, however, the number of cam lobes spaced around the periphery of the cam shaft must also increase. As the number of cam lobes spaced around the periphery of the cam shaft increases, the diameter of the cam shaft must be increased. The diameter of the cam shaft is limited by the available space within the reprographic printing machine. As a result, the number of cam lobes and the number of separately actuatable transfer assist blade segments are likewise limited.
The few discrete transfer assist blade dimensions available in the prior art devices may not always correspond to the dimension of the print sheets being processed for imaging in a reprographic printing machine. For example, a reprographic printing machine may be provided with a transfer assist blade having variable segmented lengths corresponding to print sheets having cross-process dimensions or width of 11xe2x80x3, 11.7xe2x80x3, 13xe2x80x3, and 14xe2x80x3. In the case where a 10xe2x80x3 paper width is to be processed through the transfer station, the 11xe2x80x3 blade segment is actuated. As a result, an inch of the transfer assist blade contacts the surface of the photoreceptor. The area of the blade that contacts the photoreceptor will, in most instances, pick up residual dirt and toner from the photoconductive surface. The next job run which processes print sheets having a dimension greater than 10xe2x80x3 will have the residual dirt on the transfer assist blade transferred to the back side of the print sheet, resulting in an unacceptable print quality defect. More importantly, continuous frictional contact between the blade and the photoreceptor may cause permanent damage to the photoreceptor.
In the case of a print sheet having a dimension of, for example, 12.5xe2x80x3, the transfer assist blade segments corresponding to a print sheet dimension of 11.7xe2x80x3 may be actuated. In this case, the widthwise marginal regions of the print sheet extending beyond the 11.7 inches will not be pressed against the photoconductive surface by the transfer assist blade. As a result, the risk of transfer deletions intended to be eliminated by the transfer assist blade will not be prevented in those portions of the print sheet extending beyond the marginal regions of the transfer assist blade.
There is a need in the prior art for a variable length transfer assist blade having a large number of available lengths, in order to accommodate print sheets having a large number of different cross-process dimensions or widths. Such a transfer assist blade must fit within the limited space available in modern electrostatographic printing and copying machines. It is also may be desirable for such a transfer assist blade to be capable of switching from one width to another quickly enough to do so between pitches (i.e. in between immediately consecutive print sheets), and thereby avoid the need to skip a pitch.
An apparatus according to one form of the present invention includes a resilient contact blade having a blade root and a blade tip. The blade is movable from an inoperative position in which the blade root is spaced from a print sheet contacting an imaging member by a first distance and the blade tip is spaced from the print sheet to an operative position in which the blade root is spaced from the print sheet by a second distance that is greater than the first distance. A blade deflector located in the path of travel of the blade from the inoperative position to the operative position, wherein, while the blade is moving from the inoperative position to the operative position the blade engages the deflector. When the blade is in the operative position the blade is deflected by the deflector causing the blade tip to contact the print sheet and press the print sheet against the imaging member.
An apparatus according to another form of the present invention includes a contact blade, formed of a plurality of blade segments, mounted parallel to and spaced from an imaging surface. A plurality of blade lifters, one blade lifter for each of the blade segments, are individually movable from an inoperative position immediately adjacent to the blade segments to an operative position. When in the operative position the lifters engage the blade segments and deflect the blade segments causing tips of the blade segments to contact a the print sheet contacting the imaging surface and press the print sheet against the imaging surface. A lifter activating device for moving a current select number of adjacent blade lifters into the operative position. The current select number being selected such that a current number of adjacent blade segments having a cumulative length that is equal to a width of a current print sheet contacting the imaging surface are deflected and contact with the current print sheet. A lifter locking member for engaging the current select blade lifters in the operative position and current non-selected blade lifters in the inoperative position while the current print sheet is in contact with the imaging surface.
Another form of the present invention includes a contact blade mounted parallel to and spaced from an imaging surface, the contact blade being formed of a plurality of blade segments. A plurality of blade lifters, one blade lifter for each of the blade segments, are individually movable from an inoperative position immediately adjacent to the blade segments to an operative position in which the lifters engage the blade segments. In the operative position the lifters deflect the blade segments causing tips of the blade segments to contact a print sheet contacting the imaging surface and press the print sheet against the imaging surface. A guideway extending along ends of the blade lifters remote from the contact blade. An elongate cam slidably mounted in the guideway, the cam having gear teeth formed along one side thereof. A pinion gear mounted adjacent to the guideway in engagement with the gear teeth on the cam. A motor operatively connected to the pinion gear for rotating the pinion gear, moving the cam in the guideway, and thereby moving a select number of the blade lifters into the operative position. The select number being selected such that a select number of adjacent blade segments having a cumulative length that is equal to a width of the print sheet contacting the imaging surface are deflected and contact the print sheet.